Maintaining a minimum bandwidth is often a significant concern in many applications involving integrated circuitry, particularly in high frequency communication systems that utilize limiting amplifiers. The bandwidth of an active inductor-loaded amplifier (or buffer) is generally a function of the total output capacitance and resistance, along with an inductance “L” which, for an active inductor, is in turn a function of a resistance “R”, parasitic capacitance, and transconductance of the transistor utilized to implement the active inductor. In the prior art, attempts to provide a predetermined bandwidth have typically consisted of utilizing a selected, fixed resistance value, which is set to tune the active inductor such that a portion of the capacitive load is canceled (and the bandwidth is extended).
Such use of a selected, fixed resistance value is disclosed in Hayashi et al., U.S. Pat. No. 5,726,613, issued Mar. 10, 1998, which provides an active inductor using a common-gate cascode arrangement of field-effect transistors (FETs), and which provides for altering the frequency characteristics of the inductor by selecting a resistance value R0, presumably during integrated circuit (IC) fabrication (Cols. 10–11).
Another active inductor is disclosed in Leifso et al. US. Pat. No. 6,211,753 B1, issued Apr. 3, 2001, which also utilizes a cascode topology incorporating two tuning capacitors (having significantly greater capacitance than the FET parasitic capacitances). Actual tuning, however, was accomplished manually, by breaking air bridges within sub-capacitors comprising the two capacitors (Col. 6).
Process variations in integrated circuit manufacture, temperature variations, and supply variations, however, often significantly affect the resulting resistance, capacitance, and transconductance of an active inductor circuit. As a consequence, the resulting available bandwidth is also subject to wide variations, despite any selection of resistance or capacitance values for fabrication.
A need remains, therefore, for an active inductor circuit which is capable of being tuned continuously to achieve a desired bandwidth, over process, temperature and supply variations. Such an active inductor circuit should provide such capability for real-time and automatic control over the inductance, through use of a third terminal which is couplable to other circuitry for real-time feedback and control. Such a third terminal should also be adjustable open-loop to enhance system performance through use of a digital-to-analog converter, depending upon the desired level of inductance.